1. Field of the Invention
This invention relates to a draft tube for hydraulic turbines. More particularly, this invention relates to a draft tube having means to avoid boundary layer separation along the draft tube wall.
2. Description of the Prior Art
In hydraulic turbine installations, draft tubes are well known for directing the flow of water from the turbine runner to a discharge reservoir. The draft tube is designed to recover, by diffuser action, a major portion of the kinetic energy of the water delivered to the draft tube.
The cross-sectional area of draft tubes is divergent. That is, the cross-sectional area increases from an upstream end near the turbine runner to a downstream end near the discharge reservoir. The rate of divergence must not exceed certain limits in order to avoid a phenomena whereby water flow within the draft tube will lose contact with the draft tube wall. This phenomena is commonly referred to as boundary layer separation and is undesirable since it results in a loss of energy. As taught in U.S. Pat. No. 1,467,168 to Kaplan, dated Sept. 4, 1923, the rate of divergence (expressed as the included angle of an equivalent cone of the draft tube) is preferably no more than 10.degree.. Accordingly, to prevent boundary layer separation, draft tubes, having a predetermined cross-sectional area at their discharge ends, need be of substantial length to stay within the divergence limit. The substantial length materially adds to the cost of construction of the draft tube and loss of energy by reason of friction between the fluid and the draft tube wall.
The aforementioned U.S. Pat. No. 1,467,168 teaches an elbow-shaped draft tube which operates to deflect the flow of water and force the fluid flow across the width of the draft tube to contact the draft tube walls.
Apparatus for controlling boundary layer separation are known in arts other than hydraulic turbine installations. For example, U.S. Pat. No. 4,132,499 dated Jan. 2, 1979, teaches a wind driven energy generating device having a shroud downstream of rotor blades with air channels through the shroud for injecting a high kinetic energy flow of air from exterior the shroud into the boundary layer. U.S. Pat. No. 4,098,073 to Adkins et al dated July 4, 1978, discloses a diffuser for a gas turbine with the diffuser having first divergent duct connected by a sudden enlargement of flow area to a second divergent duct. Vortices which develop within the area of the enlargement operate to diffuse the flow and avoid boundary layer separation. U.S. Pat. No. 2,808,197 to Forgo dated Oct. 1, 1957, discloses a fan assembly for air cooled condensers of steam power plants. The fan is provided with a shroud downstream of the fan blades having openings for permitting air to pass from the shroud to immediately upstream of the fan blades.